Inorganic Crystallography

Venceslav Kaucic

National Institute of Chemistry and University of Ljubljana, 1000 Ljubljana, Slovenia

Recent gains in synchrotron X-ray intensity and brilliance, achieved as a result of technical advances in the design of insertion devices, and improvements in X-ray detectors design and data handling have opened a wide range of new opportunities in the field of inorganic crystallography. A variety of new and improved methods have appeared in powder diffraction, microcrystallography, anomalous dispersion, time-resolved and in-situ studies. Diffraction techniques employing advanced synchrotron and neutron sources have been used extensively in the study of atomic structures, charge and spin densities, reactivity, and phase transformations of inorganic materials with complex stoichiometries, such as high-T_c superconductors, some catalysts and optical materials, ferroelectrics and solid-state electrolytes.

A rapid development of ab initio X-ray powder diffraction has been conditioned by accurate high-resolution powder data. Currently, the most complex structures solved ab initio from powder data contain 40 to 60 atoms in the asymmetric unit, the number that represent most of the known inorganic structures. Many groups in the world work on the development of the methods to extend this limit. While the use of traditional direct and Patterson methods is being adapted to overcome the overlapping problem, methods based on the active use of prior structural and/or chemical information and/or utilising the power of contemporary computers are also intensively being developed. Quality of results obtained with advanced powder- and micro single-crystal diffraction methods is already comparable.

Anomalous dispersion methods, which employ the tuneability of synchrotron X-ray sources, have proved to be very successful in the structure determination of many inorganic materials composed from elements adjacent in the periodic table, and occupying the same crystallographic sites, sometimes with partial occupancy of less than 10%. The resonant X-ray techniques have already surpassed comparable neutron studies, especially for materials with prohibitively high neutron absorption cross-section and in many cases where the difference in neutron scattering lengths between elements of interest is small. The great success of the anomalous dispersion experiments, especially for powder samples, has been due to much higher speeds and dynamic ranges of the new area detectors, and that has also been the case in energy dispersive diffraction or Laue experiments. The development of Laue techniques in the recent years has enabled a wide range of sample environments to be studied and data to be collected extremely rapidly by using synchrotron X-ray or neutron sources. In recent time-resolved in-situ studies of some catalysts at elevated temperatures the entire powder diffraction patterns have been continuously monitored every few seconds during the course of catalysts activation, operation and final inactivation.